To accommodate your developing baby, your uterus gets bigger as your pregnancy progresses. The uterus first enlarges upward and then outward. This expansion pushes the center of your diaphragm out of its normal position, so this dome-shaped muscle under the lungs moves upwards about 4 centimeters, or 1 ½ inches. As the diaphragm moves upward, there is less room for your lungs to expand, and the amount of oxygen your lungs can take in is reduced. These changes can leave you feeling short of breath.